1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same.
2. Background Art
In general, a memory cell of a flash memory includes a substrate, a first gate insulator, a floating gate, a second gate insulator, and a control gate. The first gate insulator is often called tunnel insulator, and the second gate insulator is often called interlayer dielectric or interpoly dielectric.
The flash memory has a problem in that an electric field tends to concentrate on edges of an interface between the substrate and the first gate insulator and on edges of an interface between the first gate insulator and the floating gate. This is because the electric field tends to concentrate on sharp portions. The flash memory also has a problem in that an electric current tends to flow locally in edges of the first gate insulator. This is because an amount of an FN current flowing through the first gate insulator mainly depends on the electric field.
When the electric current flows locally in the first gate insulator, electron traps tend to occur in the first gate insulator. Detraps of trapped electrons are equivalent to removal of electrons from the floating gate. Therefore, when electron detraps occur, threshold voltage of the memory cell fluctuates. This makes it difficult to maintain a charge holding characteristic of the memory cell. Moreover, when miniaturization of the flash memory advances, an amount of electrons held by the floating gate decreases. Therefore, when the flash memory is miniaturized, influence of electron detraps on the threshold voltage increases. Therefore, when miniaturizing the flash memory, it is necessary to prevent the concentration of the electric field on the edges of the substrate and the floating gate.
The electric field concentration on these edges can be prevented by forming bird's beaks at the edges so as to round the edges (JP-A H6-310731 (KOKAI)). Such bird's beaks can be formed by oxidizing surfaces of sidewalls of a shallow trench after the shallow trench is formed. Thereby, the bird's beaks are formed at the edges of the substrate and the floating gate. However, such bird's beaks cause a decrease and variation of the capacitance of the first gate insulator.
Methods for solving this problem are disclosed in JP-A H7-249697 (KOKAI), JP-A H6-177392 (KOKAI), and JP-A H9-232454 (KOKAI). According to the method in JP-A H7-249697, an oxidization amount of the edges is controlled by giving a density gradient to doping density of the floating gate. According to the methods in JP-A H6-177392 (KOKAI) and JP-A H9-232454 (KOKAI), formation of bird's beaks is suppressed by adding an oxidization preventing substance to the first gate insulator. However, if formation of bird's beaks is suppressed by these methods, the problem of the local flow of the electric current in the first gate insulator cannot be solved.
The floating gate is often formed of polysilicon. Usually, an oxide layer formed by oxidizing the polysilicon contains a large number of traps. When an electric current flows in such oxide layer, electron traps tend to easily occur. Such electron traps increase electron detraps from the first gate insulator. Therefore, it is desirable to suppress the oxidization of the polysilicon as much as possible.